Shimmerleafs only appear around naturally-spawned silverwood trees. (Not when a silverwood is grown from a sapling.) The flowers drop normally but can be crafted into quicksilver. A shimmerleaf flower can be crucible-crafted into the Taint-purifying Ethereal Bloom.
The strange, glowing flowers only grow in the shadow of Silverwood Trees. Instead of nectar, these flowers produce quicksilver.
Before version 4.11, they did appear from sapling-grown trees, but dropped as quicksilver unless harvested with shears.